Yuri Milner, the Russian billionaire who backed online hits including Facebook and Twitter, is putting up $100 million in a new effort to find life on other planets. The Breakthrough Starshot project announced Tuesday by Milner and famed physicist Stephen Hawking plans to find a way to send a tiny robot spacecraft weighing a little more than a piece of paper to the Alpha Centauri star system, which is 25 trillion miles away.